This invention relates to cathode sputtering and more particularly to the configuring, utilizing and cooling of sputtering targets used in sputter deposition processes and to the cooling of such targets in an assembly in a sputtering cathode of a sputtering machine.
In sputter deposition processes, substrates are placed adjacent to a sputtering target in a processing chamber and the pressure in the chamber is reduced to a high vacuum pressure level. A negative voltage is applied to the target to produce a plasma discharge, which is often intensified and confined over the target surface by the application of a magnetic field. The plasma production creates large quantities of positive ions in the sparse gas within the chamber that bombard the target and thereby dislodge atoms or small particles of target material from the surface of the target.
The ionic bombardment of the target surface transfers energy to the target, only a small percentage of which is imparted to the dislodged atoms or particles. Generally, about ninety percent of the electrical power applied to the plasma is converted to a heating of the target. With commonly used power densities in the area of 30,000 watts per square foot of target surface area, aggressive cooling of the target is required to prevent the melting or cracking of the target and to protect the target supporting cathode assembly and adjacent structures from overheating.
The cooling technique typically used in the prior art for removing heat from the target employs a flow of water against the back face of the target or against the backing plate to which the target is bonded in a thermally conductive manner. In some cathode assembly designs, the target supporting structure of the cathode assembly is water cooled, to indirectly cool the target, which is thermally attached in the cathode assembly, by conduction. In prior art cathode designs in which a target having a concave cone shaped target surface is employed, a thick target periphery or edge allows some degree of target cooling by heat flow at the periphery of the target. One such target is disclosed in U.S. Pat. No. 4,855,033, in which the primary cooling surface of the target is around the target periphery, where cooperating surfaces of the target and supporting target nest intermesh for enhanced heat conductivity. However, this deep cone shaped target, which erodes in a narrow ring, is less ideally suited for the coating of step shaped three dimensional surface features on objects like semiconductor wafers. Furthermore, the cooling of other types of targets that relies on thermal conduction between the target and the target nest is limited by mechanical gaps that develop between the nest and target as mechanical distortions occur at high operating power. Such gaps can result in a reduction in the heat transfer outward from the target, which results in failures such as by the melting of the target.
More recent target designs, however, while providing improved performance in many respects and added coating capabilities, use targets that are thinner in relation to the sputtering surface dimensions, making the rear face of the target or of the target backing plate the primary surface suitable for cooling.
In many cathode designs, magnet structures are located behind the target rear face to shape and intensify the sputtering plasma. Often the magnet structure includes rotating magnets and associated structure that occupy much of the area of the rear target face. Examples of rotating magnet cathode assemblies are disclosed in U.S. Pat. Nos. 5,130,005, 5,252,194 and 5,242,566. Such target and cathode assemblies can be eroded in a controlled manner over the entire front face of the target providing improved coating of microscopic three dimensional features on the substrate surface. In many cases, the provision of such magnet assemblies results in design compromises between the magnet structure and the cooling capabilities, which limit the performance or reliability of the cathode and target as a unit.
In a rotating magnet apparatus, for example, cooling water has been made to flow in an inlet into a cavity behind the target and across the rear face of the target to an outlet. In such apparatus, the cooling of the target may be enhanced by the motion of the cooling water imparted by the rotary motion of a magnet carrier that rotates fully immersed in the cooling fluid in the cavity. However, the desired shape of target erosion is controlled by complexly shaped magnet structure which is rotated in bearings driven by drive gears, an input shaft, a drive belt and a motor. This immersion of components in cooling water can, over time, result in corrosion and degradation of the performance of the apparatus and a shortened life. Further, replacement of the depleted target can result in exposure of the cooling water cavity, as the target or backing plate is detached from the assembly. Such exposure can result in the introduction of small amounts of water, a primary contaminant of sputtered films, into the sputtering chamber.
For the reasons stated above, there is a need for a more effective and efficient structure for cooling a sputtering target.
It is a primary objective of the present invention to provide a sputtering cathode assembly in which a target can be effectively cooled without interfering with access to the target that is needed by magnet structure and other sputtering performance affecting components of the assembly. It is a particular objective of the present invention to provide cooling for a sputtering cathode assembly without intruding on space required by rotating magnet components and while maintaining the magnet components in isolation from cooling fluid. It is a more particular objective of the present invention to provide such a sputtering cathode assembly in which magnets, bearings and other motion drive components in particular are isolated from the cooling fluid. It is a still further objective of the present invention to provide a sputtering cathode assembly in which the cooling fluid is contained to prevent contamination of the processing chamber, particularly during removal and replacement of the sputtering target, and particularly while providing direct contact between the target and the cooling fluid during operation of the sputtering apparatus.
It is another objective of the present invention to provide a sputtering target that can be maintained in direct contact with cooling fluid when mounted in a sputtering cathode assembly while containing the cooling fluid so as to prevent contamination of the processing chamber during removal and replacement of the target. It is a still further objective of the present invention to provide a sputtering target that can be maintained in direct contact with a cooling fluid but that is protected against the contaminating or corrosive effects that the cooling fluid could have on the target itself. It is yet another objective of the present invention to provide a sputtering target that can be mounted so as to maintain a seal of the vacuum processing chamber and to maintain a seal of a cooling fluid cavity, preferably without the need for bonding the target to a structural backing plate or other such member, and without excessively increasing the nonproductive amounts of sputtering grade material required in the fabrication of the target.
According to the principles of the present invention, there is provided a sputtering target, particularly a target that is relatively thin in relation to the size of its sputtering front face, that is provided with a rear face that is adapted for contact by flowing cooling fluid maintained in a cavity behind the target, when the target is mounted in a sputtering cathode assembly. The preferred embodiment of the target is provided with an annular outwardly projecting target rim, which has a forward facing front edge having a vacuum-sealing surface that is adapted to form a vacuum tight seal with the sputtering chamber, and which has a rearward facing rear edge having a cooling fluid-sealing surface that is adapted to form a seal that surrounds the cooling fluid cavity. Preferably, the target, which is preferably circular, is formed of an integral single piece of sputtering grade material, the rear face of which is adapted to be maintained in direct contact with the cooling fluid. Additionally, the surface that is in direct contact with the fluid is preferably coated or otherwise sealed from contamination by the fluid, where the target is of a material that may absorb or otherwise interact with the fluid in a disadvantageous way.
Further, in one preferred embodiment, the target rim is preferably formed integrally of the single piece of sputtering grade material with the front and rear edges thereof adapted to support or form the seals. In addition, it is preferred that the center of the rear face of the target be provided with structure by which the center of the target can be supported, so that the target is supported at both the center and the rim thereof to prevent distortion or deformation of the target during use. Preferably also the supporting structure at the center of the target is a center hub extension formed integrally of the single piece of target material.
Further in accordance with the principles of the present invention, a sputtering cathode assembly is provided in which a sputtering target, either formed of an integral piece of sputtering material, or formed of a sputtering material bonded to a backing plate to provide the rear face thereof, is adapted to lie in contact with cooling fluid in a cavity, which is formed by sealing the target at the rear edge of its rim to a rim of the cathode assembly target mounting structure. In the preferred embodiment, the cathode assembly target mounting structure includes a cavity wall segment in the form of a cooling jacket that is removably mountable onto the cathode assembly and to which the target can be sealably secured, with the target back face forming an opposing wall to the cavity. The cooling jacket structurally connects the target to the cathode assembly and is removable as a target assembly with the target when the target is removed from the chamber for replacement.
Further in accordance with the preferred embodiment of the invention, the cooling fluid cavity is removable with the assembled target and cooling jacket, leaving the inlet and outlet port structure of the cathode assembly remaining with the cathode unit when a target is removed or replaced. Upon removal of the target and cooling jacket, seals between the cooling fluid ports in the cathode unit and cooling ducts in the removed cooling jacket automatically disconnect from each other and, preferably, seal against leakage of cooling fluid therefrom. More importantly, the magnet assembly, and particularly the drive linkage by which an assembly of rotating or otherwise moveable magnets, may be used in complete isolation from the cooling fluid, which has potentially corrosive effects.
Additionally in accordance with the preferred embodiment of the invention, distortion and deformation of the target is prevented by a structural support at the center of the target that cooperates with the mounting of the target around the target rim to hold the target firmly in a plane. The center support is preferably threaded into the target material or threaded into a central hub that is rigidly secured to the rear of the target material at the center of the target. The central support, which may also serve as a central axis about which a rotating magnet assembly may rotate, is further mounted at the back end thereof to structure that is rigid relative to the frame of the sputtering apparatus and to the chamber opening rim against which the target rim is sealed. This rigid attachment of the central support resists deformation of the target into the cavity due in part to the pressure gradient from the cooling fluid toward the vacuum of the processing chamber and in part to thermal distortion of the target due to expansion of the hot sputtering surface of the target relative to the cooled rear face of the target.
The present invention provides the advantages of effectively cooling a sputtering target, particularly where such cooling must be carried out across the rear face of the target. The cooling allows access to the rear face of the target for such magnet assemblies as are desired to shape and intensify the plasma on the front side of the target.
The present invention further provides the advantages of allowing effective rear face target cooling with cooling fluid while allowing for the use of magnet assemblies, particularly rotating or other moving magnet assemblies, that may remain isolated from the cooling fluid.
Further with the present invention, there is provided a target that can be operated with its rear face in direct contact with cooling fluid, while the surface of the target material that is in contact with the fluid is protected from a disadvantageous interaction with or contamination by the fluid. A separate backing plate may be eliminated in some embodiments of the invention by the provision of a target rim that utilizes only a small amount of sputtering grade material but seals directly against the sputtering chamber wall and the cooling fluid cavity.
Further, the invention provides the advantage of allowing removal of a sputtering target for replacement, where the target is cooled by direct contact with the cooling fluid, without allowing leakage of the fluid that can cause contamination of the sputtering chamber.
The present invention provides the further advantages of rigidly supporting a relatively thin target against pressure gradient and thermal deformation, which is particularly advantageous with the trend to targets of larger and larger diameter.
These and other objectives of the present invention will be more readily apparent from the following detailed description of the preferred embodiment of the invention in which: